SiCl4 slurry has the following properties.
1. Silicon tetrachloride is expressed by a formula of SiCl4, and an industrial abbreviation of STC. It is a fuming transparent liquid which is colorless or light yellow, and has a choking smell. It is an important material for fabricating silicate esters, organic silicon monomer, silicone oil, high temperature insulating material, silicone resin, silicone rubber, etc. In the industry, silicon tetrachloride is primarily formed by reaction between ferrosilicon and chlorine above 200° C., and then distillation of the reaction products.
2. Silicon tetrachloride can be hydrolyzed in humid air to form silicic acid and hydrogen chloride, which is accompanied with white smoke. It may present intense stimulation to eyes and upper respiratory tract. At a high concentration, it may cause corneal opacity, airway inflammation, or pulmonary edema. Its contact with skin may cause tissue necrosis.
3. SiCl4 slurry refers to a liquid which is produced during polysilicon production and has a primary volatile matter of SiCl4 (the highest SiCl4 content may be up to 98%). A certain amount of other chlorosilane (Chlorosilane is a generic terms of SiCl4, SiHCl3, SiH2Cl2, SiH3Cl) may be further comprised, such as SiHCl3 and SiH2Cl2 (Both of them are liquid state). Since the slurry may comprise a small amount of metal ions (primarily comprising [Al]+, [Fe]+, [Cu]+, [Zn]+, [Ni]+, [Mn]+, etc) and may even comprise solid particles (silica powder, silicon dioxide, etc), these metal ions and solid powder will pose significant effect on the subsequent processing sections, and thus should be removed. Even each batch of slurry may differ in proportion of constituents; the slurry is still a liquid which primarily comprises SiCl4.
The current disposal methods in the world for SiCl4 slurry produced by polysilicon production equipment are basically the same, and mainly include the following types.
In the first method, the slurry is concentrated and then dried. After removing a large amount of SiCl4 liquid, the slurry is subject to hydrolysis process. Due to the characteristics of SiCl4, concentration usually involves intermittently removing a large number of silicon tetrachloride and a small amount of trichlorosilane from the slurry with a rake dryer. Generally, the volatile matter in the slurry is reduced in concentration of 40% or lower. Then, solid slurry is discharged into a hydrolysis tank and subject to hydrolysis under stirring. This process is simple, performs drying and hydrolysis in an intermittent manner, and has a limited processing capability. Meanwhile, when the slurry is discharged into the hydrolysis tank, large amount of materials instantly fall into the water, a lot of heat is instantly generated, which vaporizes water and produces steam explosion. The water vapor generated by steam explosion dissolves hydrogen chloride vapor to form hydrochloric acid mist. The mist easily falls from the blanking mouth into the drying equipment, thus corroding the drying equipment. If a material which can not fall into the hydrolysis tank is present in the dryer, a hydrolysis reaction between the materials remained in the dryer and water vapor will also occur in the dryer, resulting in the occurrence of risk. At the same time, in this method, the flux of materials is relatively large, it is difficult to control drying, the producing capability is low, the environment may be too dry, and metal or solid powder may easily fly once it is over dry, and block the subsequent filtrating equipment gently, weigh re-enter the chlorosilane liquid which has been separated, resulting the failure of separation.
In the second method, SiCl4 slurry is directly hydrolyzed to collect silicon dioxide. In this way, water consumption is large. At the same time, in order to neutralize chlorine hydride in the hydrolyzate, consumption of lime milk or alkaline liquid is also large. The silicon dioxide collected in this method also contains some impurities, and the utilization rate of SiCl4 is very low. With the development of chlorosilane hydrogenation process and improvement of national environmental requirements, this method has been gradually become obsolete.